Oxidative Cyclization of Iodo Ketones. Synthesis of ... - ACS Publications

Sep 20, 1996 - Synthesis of 6-Oxa-5α-pregnane 2. The regioselective deprotection of the 19-hydroxy group in compound 6 was attempted with different ...
0 downloads 0 Views 259KB Size
Download PDF
J. Org. Chem. 1996, 61, 6673-6677

6673

Oxidative Cyclization of Iodo Ketones. Synthesis of 6-Oxa-5r-pregnane-3,20-dione Daniel Nicoletti, Alberto A. Ghini, and Gerardo Burton*,† Departamento de Quı´mica Orga´ nica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabello´ n 2, Ciudad Universitaria, 1428 Buenos Aires, Argentina Received January 17, 1996X

The steroidal δ- and γ-iodo ketones 1 and 9 were converted to the cyclic hemiketals 3 and 10, by oxidation to the iodoso derivatives with m-CPBA. Spontaneous cyclization of the latter intermediates to the corresponding oxocarbenium ions, followed by stereoselective addition of water, rendered the hemiketals. Depending on the reaction conditions, the five-membered oxocarbenium ion derived from the γ-iodo ketone 9 may add H2O or m-CPBA to give either the hemiketal or a BaeyerVilliger type product 12, while the oxocarbenium derived from 1 gives exclusively the hemiketal. When the reaction was carried out in dry methanol, methyl ketals were formed. Use of this methodology allowed us to synthesize 6-oxa-5R-pregnanes with and without functionalization at C-19. The replacement of one or more carbon atoms of the steroid nucleus by heteroatoms gives rise to marked changes in the biological activity of steroidal derivatives.1 Thus, numerous studies deal with total and partial syntheses of heterosteroids.2 Recently, we described a synthetic approach to 6-oxa-5β-pregnanes in which the key step was the reductive cyclization of the δ-iodo ketone moiety in secosteroid 1.3 Formation of the less stable cis isomer at the A/B ring junction was due to the steric hindrance toward hydride attack from the R face in secosteroid 1. In order to obtain the 6-oxapregnanes with trans configuration at the A/B ring junction (e.g., 2), a different cyclization approach was required.

Scheme 1

of a steroidal oxocarbenium ion. Iodine in a higher oxidation state provides a better leaving group than univalent iodine,5 and the highly labile iodoso intermediates have been postulated to be the initial products in the oxidation of alkyl iodides with m-CPBA.6 Their fate depends on the type of substrate and the solvent, affording the product of R-I bond cleavage by elimination, substitution, R-carbon oxidation, or rearrangement.7 Results and Discussion

Kishi et al.4 have shown that the stereochemical course of nucleophilic attack on a pyranosic oxocarbenium ion may be rationalized on the basis of stereoelectronics and steric effects. Thus, the oxocarbenium ion preferently accepts nucleophiles from the R (axial) side due to the anomeric effect of the ring oxygen. If a pyranosic oxocarbenium ion is formed in ring B of the steroid nucleus, nucleophiles (e.g., H2O, methanol, hydride, etc.) will attack at C-5 from the R face giving an A/B trans steroid (PM3-calculated structures are shown in Scheme 1); in this case, steric hindrance on the β face would favor stereoselectivity. We initially focused our attention on the δ-iodo ketone moiety present in secosteroid 1 as a possible precursor †

Tel/Fax: 54 1 788-6915. e-mail: [email protected] Abstract published in Advance ACS Abstracts, August 1, 1996. (1) Suginome, H.; Yamada, S.; Wang, J. B. J. Org. Chem. 1990, 55, 2170. (2) Huisman, H. O. Angew. Chem., Int. Ed. Engl. 1971, 10, 240. (3) Nicoletti, D.; Ghini, A. A.; Brachet-Cota, A. L.; Burton, G. J. Chem. Soc., Perkin Trans. 1 1995, 1089. (4) (a) Lewis, M. D.; Cha, J. K.; Kishi, Y. J. Am. Chem. Soc. 1982, 104, 4976 and references cited therein. See also: (b) Deslongchamps, P. Stereoelectronic Effects in Organic Chemistry; Pergamon Press: Oxford, 1983, pp 40-41. X

S0022-3263(96)00108-9 CCC: $12.00

Cyclization of Secosteroid 1 to an A/B TransFused Oxasteroid. We wondered if the oxygen in the carbonyl group at C-5 in secosteroid 1 could displace the C-7 iodine leaving group. In that case, a pyranosil oxocarbenium ion would be formed which, in the presence of water, would yield hemiketal 3 with the desired stereochemistry at C-5 (Scheme 2). Oxidation of iodine compounds with peroxyacids has been the subject of several studies,6,7 but to our knowledge it has not been used for hemiketal formation. Hemiketal formation was also achieved by this procedure with the γ-iodo ketone moiety present in 18-iodo-20-keto steroids (see below). Table 1 summarizes the different conditions assayed for reaction of secosteroid 1 with m-CPBA. When 2 equiv of the peracid (more than the theoretical requirement for alkyl iodide oxidation6) was used in dichloromethane saturated with water (entry 1), 5R-hydroxy-6-oxasteroid 3 was isolated in 33% yield and unreacted compound 1 was recovered among other minor products. The best yield was obtained with 5 equiv of m-CPBA in Cl2CH2H2O at 0 °C (entry 5); changing the solvent to THF-H2O (5) Cambie, R. C.; Chambers, D.; Lindsay, B. G.; Rutledge, P. S.; Woodgate, P. D. J. Chem. Soc., Perkin Trans. 1 1980, 822. (6) Macdonald, T. L.; Narasimhan. N.; Burka, L. T. J. Am. Chem. Soc. 1980, 102, 7760 and references cited therein. (7) Lee J.; Oh, J.; Jin, S.-j.; Choi, J.-R; Atwood, J. L.; Cha, J. K. J. Org. Chem. 1994, 59, 6955 and references cited therein.

© 1996 American Chemical Society

6674 J. Org. Chem., Vol. 61, No. 19, 1996

Nicoletti et al.

Scheme 2

Scheme 3

Table 1. Reaction of Iodo Ketones 1 and 9 with m-CPBA entry substrate 1 2 3 4 5 6 7 8 9 10

1 1 1 1 1 1 9 9 9 9

time temp m-CPBA (equiv) solventa (h) (°C) 2 3 4 4 5 8c 5 5 5 6c

A A A B A C A B B C

2 5 5 5 7 5 6 4 3 6

rt rt 0 0 0 rt 0 rt rt rt

product (yield, %)b 3 (33) 3 (38) 3 (52) 3 (52) 3 (67) 4 (59), 5 (18) 12 (61) 12 (10), 10 (51) 10 (64) 13 (97)d

a A, Cl CH saturated with H O; B, THF-H O (2:1); C, dry 2 2 2 2 methanol. b Yields correspond to isolated products. c Dry m-CPBA d was used. This compound was partially hydrolyzed to 10 during chromatography. Yield corresponds to crude reaction product which was essentially pure by 1H NMR.

(entry 4) did not improve the yield. When the reaction was carried out in methanol under anhydrous conditions (entry 6), the main product was methyl ketal 4; the 7-methoxy derivative 5, arising by direct nucleophilic attack of methanol at C-7 of the iodoso intermediate, was formed as a minor product. The latter results are consistent with the formation of the intermediate pyranosyl oxocarbenium ion postulated above, as the major reaction path. The structures of hemiketal 3 and methyl ketal 4 and their stereochemistry at position 5 were confirmed by 1H and 13C NMR (proton decoupled and DEPT). The 3-HR resonance (δ 5.05 for 3 and δ 4.78 for 4) was especially diagnostic of the axial position of this hydrogen, indicative of the chair conformation of the A ring in an A/B trans-fused steroid. The carbon resonance at 97.3 ppm, which is typical of a hemiketal carbon assigned to C-5, confirmed the structure of compound 3. This resonance was shifted to 99.7 ppm in methyl ketal 4. Stereoselective Reduction of Hemiketal 3. The conversion of hemiketal 3 to oxasteroid 6 with ZnBH4/ TMSCl in DME at 0 °C8 occurred in low yield (30%). However a 75% yield was attained with Et3SiH/BF3‚Et2O (Cl2CH2, -15 °C, 1 h) (Scheme 3).9 As above, the attack of the nucleophile on the oxocarbenium intermediate (in (8) Kotsuki, H.; Ushio, Y.; Yoshimura, N.; Ochi, M. J. Org. Chem. 1987, 52, 2594. (9) The method used was essentially a hybrid of several reported ones. For related examples, see: Kraus, G. A.; Frazier, K. A.; Roth, B. D.; Taschner, M. J.; Neuenschwander, K. J. Org. Chem. 1981, 46, 2417 and Lewis et al. in ref 4.

this case hydride) occurred from the less hindered R face with stereoselective formation of the 5R-H-6-oxasteroid 6. The structure of compound 6 was confirmed by 1H and 13C NMR spectroscopy; the stereochemistry at C-5 was evident from the J values in the 1H NMR spectrum. Two resonances were of special diagnostic value: that at 3.10 ppm (dd, J ) 12.8 and J ) 3.6 Hz) was attributed to the 5-HR and that at 4.76 ppm assigned to 3-HR. Final confirmation of the stereochemistry at C-5 was carried out at a later stage on compound 8 based on its NOESY spectrum, as all the oxapregnanes synthesized had identical configuration at this position (see below). Synthesis of 6-Oxa-5r-pregnane 2. The regioselective deprotection of the 19-hydroxy group in compound 6 was attempted with different methods ranging from basic (KHCO3 in methanol or methanol-water) to acidic conditions (TsOH in methanol). In all cases, mixtures of products with partial deprotection of the 3-hydroxy group resulted.10 The desired product 7 was finally obtained in good yield (96%) by reduction with NaBH4 at -10 °C; under this very mild and controlled condition, the 3-acetate was not affected even when a very large excess of reagent (ca. 15:1) was used. As in the 5-β series,3 deoxygenation of the primary hydroxyl group attached at C-19 was carried out using the Barton procedure, by reduction of the 19-dithiocarbonate with diphenylsilane which gave 8 in 80% yield (56% from 7).11 The NOESY spectrum of the latter compound gave us clear evidence of the trans-fusion of rings A and B, showing strong NOEs of 3-HR, 7-HR, and 9-HR with 5-HR and of the 10-methyl hydrogens with 4-Hβ and 8-Hβ (Figure 1). Deacetylation with LiAlH4 to the diol, and oxidation with PCC on alumina,12 yielded 6-oxa5R-pregnane-3,20 dione (2) in 22% overall yield from 1. (10) With KHCO3, diacetate 7 was obtained predominantly (ca. 70% yield), but with TsOH the 3,19-diol was the major product. (11) Barton, D. H. R.; Blundell, P.; Dorchak, J.; Jang, D. O.; Jaszberenyi, J. C. Tetrahedron 1991, 47, 8969 and references cited therein. In the 5β-series, tributyltin hydride was used for reduction of the dithiocarbonate to the analogous 6-oxa-5β-pregnane in 63% yield (ref 3). However, when diphenylsilane/benzoyl peroxide was used, the yield increased to 82%; thus the latter method was used in the present preparation. (12) Cheng, Y.-S.; Liu, W.-L.; Chen, S.-h. Synthesis 1980, 223.

Oxidative Cyclization of Iodo Ketones

Figure 1. Most stable conformer (PM3, AMPAC 5.0) of oxasteroid 8, showing relevant observed NOEs.

Oxidative Cyclization of 18-Iodo-20-ketopregnanes. To determine the scope of the oxidative cyclization of iodo ketones used above, we tried this reaction on steroids having a γ-iodo ketone moiety. Cyclization of 18-iodo-20-ketopregnanes to the corresponding hemiketals with silver acetate is a well-known procedure which constitutes a key step in the preparation of several steroids of biological significance.13 As an alternative procedure and based on the results obtained above, we tried this cyclization with m-CPBA. It should be noted that silver ion promoted cyclization of secosteroid 1 ocurred in very low yields. When 18-iodoprogesterone (9) was reacted with mCPBA under the same conditions as those used for the preparation of 3 (Table 1, entry 7), a single product was obtained in 61% yield. This compound was identified as 18-acetoxytestosterone (12) on the basis of its 1H and 13C NMR spectra. It should be noted that the latter compound has structural features that make it a suitable precursor for naturally occurring steroids of biological importance.14 When the reaction was stopped after only 1 h, a transient species could be detected in the 1H NMR spectra of the reaction mixture showing typical resonances for aromatic hydrogens of a m-CPBA ester and an 18-oxygenated steroid. Attempts to purify this compound by chromatography were unsuccessful, as it decomposed on contact with the adsorbent (compound 12 resulted from the attempted purification of this intermediate over silica gel). The structure of the intermediate was tentatively assigned as the peroxyester 11, similar to those formed in the Baeyer-Villiger reaction.15 A plausible mechanism for the formation of these compounds is depicted in Scheme 4 (path a) in which the peroxyacid attacks the furanosic oxocarbenium ion intermediate in a Baeyer-Villiger like reaction. The regioselective cleavage of the intermediate cyclic ion derived from 11 may be explained by assuming that the steric bulk of the electrophile H+ is small and thus favors thermodynamic protonation of the oxygen on a secondary carbon.16 (13) (a) Woodward, R. B.; Brutcher, F. V. J. Am. Chem. Soc. 1958, 80, 209. (b) Choay, P.; Monneret, C.; Khuong-Hun Q. Bull. Soc. Chim. Fr. 1973, 1456. (c) Kirk, D. N.; Rajagopalan, M. S. J. Chem. Soc., Perkin Trans. 1 1975, 1860. (d) For a related approach via 18-nitrates, see: Barton, D. H. R.; Day, M. J.; Hesse, R. H.; Pechet, M. M. J. Chem. Soc., Perkin Trans. 1 1975, 2252. (14) For example, the neonatal metabolite 3β,16R,18-trihydroxy-5androsten-17-one and related compounds. Das, R.; Kirk, D. N. J. Chem. Soc., Perkin Trans. 1 1984, 1821. (15) March, J. Advanced Organic Chemistry, 4th Ed.; John Wiley & Sons: New York, 1992; pp 1098-1099. (16) Compound 12 is not the thermodynamic product obtained by acyl transfer. On standing in Cl2CH2 in the presence of catalytic amounts of acid, 12 yields a 1:1 mixture of 12 and the 17-acetoxy-18hydroxy isomer.

J. Org. Chem., Vol. 61, No. 19, 1996 6675

From the above mechanism, it may be predicted that a high concentration of water would compete favorably with m-CPBA as a nucleophile in the addition to the intermediate oxocarbenium ion. The expected product of this reaction would be 18-hydroxyprogesterone 10 (Scheme 4, path b). Thus, when 9 was reacted with m-CPBA in THF-H2O (2:1) for 4 h at rt17 (Table 1, entry 8), a mixture of compounds 10 and 12 was obtained in a 5:1 ratio (according to the 1H-NMR spectra). Adjusting the reaction time to 3 h (entry 9) allowed us to obtain 10 in 64% yield. In a similar way when the reaction was carried out in methanol under anhydrous conditions (entry 10) the methyl ketal 13 was obtained. No 18methoxy derivatives were obtained in the latter case, as the neopentylic C-18 is highly hindered toward intermolecular nucleophilic attack. Conclusions. Cyclic hemiketal formation from γ- and δ-iodo ketones has been achieved with the intermediacy of iodosyl species. The resulting oxocarbenium ions react with available nucleophiles (e.g., H2O, methanol, mCPBA) to give hemiketals, ketals, or Baeyer-Villiger type products depending on reaction conditions, ring size, and/or steric hindrance. This new methodology allowed us to synthesize 6-oxa-5R-pregnanes with excellent stereoselectivity.

Experimental Section Melting points are uncorrected. IR spectra were recorded in KBr pellets. 1H and 13C NMR spectra were measured at 200.13 and 50.32 MHz, respectively, in CDCl3. Chemical shifts are downfield from TMS as internal standard, and J values are in hertz. NMR assignments, when given, are based on DEPT and 2D spectra (COSY-45, NOESY and HETCOSY). The electron impact mass spectra were obtained at 70 eV by direct inlet. Semiempirical calculations were performed with AMPAC 5.0 (Semichem). All solvents used were reagent grade. Solvents were evaporated at ca. 45 °C under vacuum. Dry m-CPBA was prepared by dissolving 80% m-CPBA in anhydrous Cl2CH2, drying with anhydrous Na2SO4 (4 h), evaporation of the solvent under a stream of N2, and drying of the residue under vacuum (1 h, rt, 5 × 10-4 Torr). 3β,20βDiacetoxy-19-(formyloxy)-7-iodo-6-nor-5,7-secopregnan-5-one (1) was prepared from pregnenolone acetate (3β-acetoxypregn5-en-20-one) in 50% yield, as single product, using the procedure described previously3 followed by chromatography on octadecyl-functionalized silica gel (Aldrich) with methanolwater as eluent. 18-Iodopregn-4-ene-3,20-dione (9) was prepared from 20β-hydroxypregn-4-en-3-one in 55% yield as described previously18 and purified by chromatography on octadecyl-functionalized silica gel (Aldrich). 3β,20β-Diacetoxy-5r-hydroxy-19-(formyloxy)-6-oxapregnane (3). To a solution of 80% m-CPBA (1.00 g, containing 4.64 mmol) in Cl2CH2 (15 mL) and H2O (0.1 mL) at 0 °C was added rapidly a solution of secosteroid 1 (0.527 g, 0.91 mmol) in Cl2CH2 (15 mL). The homogeneous solution was stirred at 0 °C for 7 h, diluted with ether, washed successively with 5% aqueous Na2S2O3, a saturated aqueous NaHCO3 solution, and brine, dried (Na2SO4), and evaporated to dryness. Chromatography on silica gel with ethyl acetate-hexane as eluent yielded hemiketal 3 (0.287 g, 67% from 1); mp 148-150 °C (from acetone-hexane). IR (cm-1): 3440, 1728, 1246, 1166, 1037. 1H NMR (δ): 0.65 (3H, s), 1.14 (3H, d, J ) 6.0), 2.00 (3H, s), 2.02 (3H, s), 3.51 (1H, dd, J ) 5.0, 11.0), 3.72 (1H, t, J ) 11.0), 4.56 (1H, d, J ) 12.8), 4.63 (1H, d, J ) 12.8), 4.83 (1H, m), 5.05 (1H, tt, J ) 5.0, 11.0), 8.12 (1H, s). 13C NMR (δ): 12.8 (CH3), 19.9 (CH3), 21.3 (CH3), 21.4 (CH3), 23.2 (CH2), 25.0 (CH2), 25.7 (CH2), 26.2 (CH2), 29.7 (CH2), 34.3 (CH), 39.3 (17) The reaction was very slow at 0 °C. (18) Benedetti, M. O. V.; Burton, G. Org. Prep. Proced. Int. 1992, 24, 701 and references cited therein.

6676 J. Org. Chem., Vol. 61, No. 19, 1996

Nicoletti et al. Scheme 4

(CH2), 39.5 (CH2), 41.8 (C), 42.7 (CH), 42.8 (C), 51.7 (CH), 54.5 (CH), 62.7 (CH2), 64.6 (CH2), 70.6 (CH), 72.7 (CH), 97.3 (C), 160.8 (CH), 170.3 (C), 170.7 (C). MS (m/z, %): 388 (M - AcOH - H2O, 40), 342 (388 - HCO2H, 19). HRMS: calcd for C23H32O5 (M - AcOH - H2O) 388.2249, found 388.2248. 3β,20β-Diacetoxy-19-(formyloxy)-6-oxa-5r-pregnane (6). A solution of hemiketal 3 (0.277 g; 0.59 mmol), triethysilane (0.975 mL, 6.1 mmol), and BF3‚Et2O (0.750 mL, 6.1 mmol) in dry Cl2CH2 (29 mL) was stirred at -15 °C under N2 for 1 h. The reaction was quenched with saturated aqueous NaHCO3 solution, the cooling bath was removed, and the reaction mixture was allowed to warm to rt with vigorous stirring. The solution was diluted with ether, washed successively with saturated aqueous NaHCO3 solution and brine, dried (Na2SO4), and evaporated to dryness. Chromatography on silica gel with ethyl acetate-hexane as eluent yielded 6 (0.200 g, 75% from 3) homogeneous by TLC; mp 165-167 °C (from acetonehexane). IR (cm-1): 1733, 1244, 1167, 1103, 1036. 1H NMR (δ): 0.66 (3H, s), 1.15 (3H, d, J ) 6.1) 2.00 (3H, s), 2.03 (3H, s), 3.10 (1H, dd, J ) 3.6, 12.8), 3.12 (1H, t, J ) 11.0), 3.91 (1H, dd, J ) 5.0, 11.0), 4.52 (2H, AB q, J ) 12.3), 4.76 (1H, m), 4.84 (1H, m), 8.11 (1H, s). 13C NMR (δ): 12.7 (CH3), 19.9 (CH3), 21.2 (CH3), 21.4 (CH3), 21.6 (CH2), 23.3 (CH2), 25.6 (CH2), 26.6 (CH2), 27.3 (CH2), 32.9 (CH2), 35.5 (CH), 38.6 (C), 39.4 (CH2), 42.7 (C), 51.6 (CH), 53.2 (CH), 54.5 (CH), 61.7 (CH2), 70.8 (CH), 72.6 (CH), 73.7 (CH2), 82.4 (CH), 160.9 (CH), 170.4 (C). MS (m/z, %): 450 (M+, 0.6), 390 (M - AcOH, 10), 344 (390 - HCO2H, 3), 330 (M - 2 AcOH, 11). HRMS: calcd for C23H34O5 (M - AcOH) 390.2406, found 390.2407. 3β,20β-Diacetoxy-19-hydroxy-6-oxa-5r-pregnane (7). A solution of 6 (0.190 g, 0.42 mmol) and NaBH4 (0.122 g, 3.23 mmol) in absolute ethanol (30 mL) was stirred at -10 °C for 3 h, acidified (pH 5) with 1 M HCl, and then neutralized with a 10% aqueous NaHCO3 solution. The solution was concentrated in vacuo to a volume of 10 mL, diluted with water, and extracted with ether. The extract was washed with water, dried (Na2SO4), and evaporated to dryness. Chromatography on silica gel with ethyl acetate-hexane as eluent yielded 7 (0.171 g, 96% from 6) homogeneous by TLC; mp 179-181 °C (from acetone-hexane). IR (cm-1): 3460, 1732, 1244, 1038. 1H NMR (δ): 0.71 (3H, s), 1.15 (3H, d, J ) 6.1), 2.01 (3H, s), 2.03 (3H, s), 3.15 (1H, dd, J ) 4.7, 12.4), 3.16 (1H, t, J ) 11.0), 3.78 (1H, d, J ) 11.9), 3.92 (1H, dd, J ) 5.0, 11.0), 4.12 (1H, d, J ) 11.9), 4.72 (1H, m), 4.84 (1H, m). 13C NMR (δ): 12.9 (CH3), 19.8 (CH3), 21.1 (CH2), 21.2 (CH3), 21.4 (CH3), 23.2 (CH2), 25.6 (CH2), 26.9 (CH2), 28.6 (CH2), 32.9 (CH2), 36.2 (CH), 38.7 (C), 39.3 (CH2), 42.9 (C), 52.1 (CH), 53.5 (CH), 54.5 (CH), 62.0 (CH2), 71.0 (CH), 72.7 (CH), 73.8 (CH2), 83.7 (CH), 170.3 (C); FAB MS (3-nitrobenzyl alcohol + KCl) (m/z, %): 461 (M + K, 100). Anal. Calcd for C24H38O6: C, 68.2; H, 9.1. Found: C, 67.8; H, 8.7.

3β,20β-Diacetoxy-6-oxa-5r-pregnane (8). To a solution of 19-hydroxyoxasteroid 7 (0.161 g, 0.38 mmol), and DBU (0.23 mL, 1.55 mmol) in DMF (2.0 mL) was added CS2 (2.1 mL, 34.8 mmol) and the reaction mixture was stirred for 45 min at rt. Methyl iodide (4.2 mL, 67.5 mmol) was added and the stirring continued for 45 min. The reaction mixture was evaporated, and the residue was diluted with water and extracted with ethyl acetate. The organic layer was washed with brine, dried (Na2SO4), and evaporated to dryness. Chromatography on silica gel with ethyl acetate-hexane as eluent yielded the 19dithiocarbonate (0.136 g, 70% from 7). 1H NMR (δ): 0.64 (3H, s), 1.15 (3H, d, J ) 6.1); 2.01 (3H, s), 2.03 (3H, s), 2.57 (3H, s), 3.12 (1H, dd, J ) 5.1, 12.1), 3.14 (1H, t, J ) 11.2), 3.91 (1H, dd, J ) 4.9, 11.2), 4.80 (2H, m), 4.85 (1H, d, J ) 12.1), 5.03 (1H, d, J ) 12.1). A solution of the dithiocarbonate (0.136 g, 0.27 mmol) in xylene (1.0 mL) was heated to 150 °C under N2, and diphenylsilane (0.250 mL, 1.35 mmol) was added followed by 11 × 0.06 mL aliquots of a solution of benzoyl peroxide in xylene (0.129 g/mL, 1.8 equiv) at 20 min intervals. The solvent was evaporated, and the residue was chromatographed on silica gel with dichloromethane-hexane as eluent yielding 8 (0.086 g, 56% from 7); mp 139-140 °C (from ethanol-water). IR (cm-1): 1732, 1244, 1117, 1053, 1034. 1H NMR (δ): 0.65 (3H, s, 13-Me), 0.92 (3H, s, 10-Me), 1.14 (3H, d, J ) 6.1, 20Me), 2.01 (3H, s, Ac), 2.03 (3H, s, Ac), 2.93 (1H, dd, J5R4R ) 3.8, J5R4β ) 12.2, 5-HR), 3.09 (1H, t, Jgem ≈ J7R,8 ) 11.0, 7-HR), 3.86 (1H, dd, J7β,8 ) 4.7, Jgem ) 11.0, 7-Hβ), 4.69 (1H, m, 3-H), 4.83 (1H, m, 20-H). 13C NMR (δ): 11.7 (CH3, C-19), 12.7 (CH3, C-18), 19.9 (CH3, C-21), 20.4 (CH2, C-11), 21.3 and 21.5 (CH3, Ac), 23.4 (CH2, C-1), 25.7 (CH2, C-15), 26.8 (CH2, C-16), 32.1 (CH2, C-2), 32.8 (CH2, C-4), 35.3 (CH, C-8), 35.9 (C, C-10), 39.1 (CH2, C-12), 42.8 (C, C-13), 51.4 (CH, C-9), 53.2 (CH, C-14), 54.7 (CH, C-17), 71.5 (CH, C-3), 72.7 (CH, C-20), 73.6 (CH2, C-7) 82.5 (CH, C-5), 170.4 (C, Ac). MS (m/z, %): 406 (M+, 2), 346 (M - AcOH, 27). Anal. Calcd for C24H38O5: C, 70.9, H, 9.4. Found: C, 70.6; H, 9.2. 6-Oxa-5r-pregnane-3,20-dione (2). A solution of diacetate 8 (0.076 g, 0.19 mmol) and LiAlH4 (0.076 g, 2.0 mmol) in dry ether (7.5 mL) was stirred at rt for 3 h under N2. The reaction mixture was treated successively with ethyl acetate and 10% aqueous HCl. The aqueous layer was extracted with ethyl acetate, and the combined organic extracts were washed with aqueous NaHCO3 and water, dried (Na2SO4), and evaporated to dryness, yielding the 3,20-diol (0.060 g, 96% from 8). 1H NMR (δ): 0.77 (3H, s), 0.92 (3H, s), 1.13 (3H, d, J ) 6.1), 2.89 (1H, dd, J ) 3.8, 12.1), 3.09 (1H, t, J ) 11.0), 3.61 (1H, m), 3.20 (1H, m), 3.87 (1H,dd, J ) 4.7, 11.0). The crude diol (0.060 g, 0.18 mmol) in dry CH2Cl2 (4.0 mL) was stirred at rt with PCC on alumina (1.1 g, 1 mmol/g, 1.1 mmol), for 4 h under N2. The resultant brown mixture was diluted with ether, percolated through Florisil, eluting with ether and Cl2CH2, and

Oxidative Cyclization of Iodo Ketones evaporated to dryness. Chromatography on silica gel with ethyl acetate-hexane as eluent yielded 2 (0.048 g, 81% from 8); mp 187-188 °C (from ethanol). IR (cm-1): 1709, 1388, 1364, 1072, 1039. 1H NMR (δ): 0.68 (3H, s, 13-Me), 1.09 (3H, s, 10-Me), 2.13 (3H, s, 20-Me), 3.19 (1H, t, Jgem ≈ J7R,8 ) 11.1, 7-HR), 3.24 (1H, t, J5R,4β ≈ J5R,4R ) 9.5, 5-HR), 3.93 (1H, dd, J7β,8 4.7, Jgem ) 11.1, 7-Hβ). 13C NMR (δ): 11.0 (CH3, C-18), 13.6 (CH3, C-19), 20.7 (CH2, C-11), 23.3 (CH2, C-15), 23.6 (CH2, C-16), 31.6 (CH3, C-21), 32.1 (CH2, C-1), 35.2 (CH, C-8), 35.9 (C, C-10), 37.5 (CH2, C-2), 38.7 (CH2, C-4), 44.1 (CH2, C-12), 44.4 (C, C-13), 52.1 (CH, C-9), 52.7 (CH, C-14), 63.1 (CH, C-17), 73.2 (CH2, C-7), 82.6 (CH, C-5), 209.0 (C, C-3 and C-20). MS (m/z, %): 318 (M+, 7), 300 (M - H2O, 7). Anal. Calcd for C20H30O3: C, 75.4; H, 9.5. Found: C, 75.2; H, 9.8. Reaction of Secosteroid 1 with m-CPBA/MeOH. 3β,20βDiacetoxy-5r-methoxy-19-(formyloxy)-6-oxapregnane (4). To a solution of dry m-CPBA (from 0.275 g of m-CPBA 80%, containing 1.27 mmol) in dry MeOH (5 mL) at rt was added rapidly a solution of secosteroid 1 (0.09 g, 0.16 mmol) in dry methanol (5 mL). The homogeneous solution was stirred at rt for 5 h and the reaction stopped by addition of saturated aqueous Na2S2O3 (2 mL). The resulting mixture was concentrated to 1/5 of the volume, extracted with Cl2CH2, washed successively with saturated aqueous NaHCO3 solution and brine, dried (Na2SO4), and evaporated to dryness. Chromatography on silica gel with ethyl acetate-hexane (1:9) as eluent yielded methyl ketal 4 (0.045 g, 59% from 1); mp 94-95 °C (from acetone-hexane). IR (cm-1): 1728, 1456, 1367, 1244, 1168, 1037. 1H NMR (δ): 0.64 (3H, s), 1.14 (3H, d, J ) 6.1), 2.00 (3H, s), 2.02 (3H, s), 2.22 (1H, ddd, J ) 1.1, 5.1, 13.7), 3.23 (3H, s), 3,27 (1H, t, J ) 11.0), 3.50 (1H, dd, J ) 5.1, 11.0), 4.61 (2H, s), 4.78 (1H, m), 4.83 (1H, m), 8.12 (1H, s). 13C NMR (δ): 12.8 (CH3), 19.9 (CH3), 21.2 (CH3), 23.2 (CH2), 24.8 (CH2), 25.7 (CH2), 26.3 (CH2), 31.9 (CH2), 34.2 (CH), 39.5 (CH2), 42.3 (C), 42.5 (CH and CH2), 42.8 (C), 47.0 (CH3), 51.8 (CH), 54.4 (CH), 63.0 (CH2), 65.0 (CH2), 69.8 (CH), 72.7 (CH), 99.7 (C), 160.8 (CH), 170.3 (C), 170.5 (C). MS (m/z, %): 480 (M+, 2), 434 (M - HCO2H, 15), 420 (M - AcOH, 100), 388 (420 MeOH, 13). Anal. Calcd for C26H40O8: C, 65.0; H, 8.4. Found: C, 64.8; H, 8.6. Further elution with ethyl acetatehexane (2:8) yielded methoxy derivative 5 (0.014 g, 18% from 1). IR (cm-1): 1730, 1373, 1247, 1169, 1118. 1H NMR (δ): 0.63 (3H, s), 1.15 (3H, d, J ) 6.1), 2.01 (3H, s), 2.03 (3H, s), 2.45 (1H, dt, J ) 14.8, 2.0), 3.06 (3H, s), 3,06 (1H, m), 3.25 (1H, dd, J ) 2.4, 10.0), 3.34 (1H, dd, J ) 4.4, 14.8), 4.31 (1H, d, J ) 11.7), 4.56 (1H, d, J ) 11.7), 4.85 (1H, m), 5.38 (1H, br s, W1/2 ) 9), 8.11 (1H, s). 13C NMR (δ): 12.1 (CH3), 19.8 (CH3), 21.2 (CH3), 21.4 (CH3), 23.1 (CH2), 24.4 (CH2), 24.7 (CH2), 25.1 (CH2), 28.1 (CH2), 39.0 (CH), 39.5 (CH and CH2), 42.2 (C), 42.9 (CH2), 51.5 (CH), 53.7 (C), 55.1 (CH), 57.6 (CH3), 64.7 (CH2), 70.4 (CH2), 72.7 (CH), 72.8 (CH), 160.9 (CH), 170.3 (C), 211.3 (C). MS (m/z, %): 480 (M+, 3), 420 (M - AcOH, 15), 388 (420 - MeOH, 100), 328 (388 - AcOH, 40). HRMS: calcd for C26H40O8 480.2723, found 480.2725.

J. Org. Chem., Vol. 61, No. 19, 1996 6677 Reaction of 18-Iodopregn-4-ene-3,20-dione (9) with m-CPBA/THF-H2O. 18-Hydroxypregn-4-ene-3,20-dione (10). A solution of 9 (0.044 g, 0.1 mmol) in THF (2.0 mL) was added rapidly to a solution of 80% m-CPBA (0.108 g, containing 0.5 mmol) in THF (2.0 mL) and water (2.0 mL) and stirred for 3 h at rt. Workup as above, followed by chromatography on Florisil with ethyl acetate-hexane as eluent, yielded 18hydroxyprogesterone (10) (0.021 g, 64% from 9); mp 157-159 °C (from acetone) (lit.18 mp 157-159 °C); 1H and 13C NMR spectra are identical to those of an authentic standard. Reaction of 18-Iodopregn-4-ene-3,20-dione (9) with m-CPBA/Cl2CH2-H2O. 18-Acetoxytestosterone (12). A solution of 9 (0.044 g, 0.1 mmol) in Cl2CH2 (2.0 mL) was added rapidly to a solution of 80% m-CPBA (0.108 g, containing 0.5 mmol) in Cl2CH2 (2.0 mL) and water (0.010 mL) at 0 °C and stirred for 6 h at 0 °C. Workup as for 3 followed by chromatography on silica gel with ethyl acetate-hexane as eluent yielded 12 (0.021 g, 61% from 9); mp 169-172 °C (from diethyl ether) (lit.19 mp 170-173 °C). 1H NMR (δ): 1.21 (3H, s), 2.10 (3H, s), 3.79 (1H, t, J ) 8.5), 4.22 (1H, d, J ) 11.8), 4.38 (1H,d, J ) 11.8) and 5.73 (1H, s). 13C NMR (δ): 17.4 (CH3), 20.5 (CH2), 21.2 (CH3), 23.4 (CH2), 30.9 (CH2), 31.7 (CH2), 31.9 (CH2), 32.6 (CH2), 33.9 (CH2), 35.6 (CH), 35.8 (CH2), 38.6 (C), 45.3 (C), 50.4 (CH), 53.9 (CH), 62.1 (CH2), 82.0 (CH), 124.1 (CH), 171.0 (C), 199.4 (C). Reaction of 18-Iodopregn-4-ene-3,20-dione (9) with m-CPBA/MeOH. 20-Methoxy-18,20-oxidopregn-4-en-3one (13). A solution of 9 (0.04 g, 0.091 mmol) in dry MeOH (2.0 mL) was added rapidly to a solution of dry m-CPBA (from 0.125 g of m-CPBA 80%, containing 0.58 mmol) in dry MeOH (2.0 mL) and stirred for 6 h at rt. Workup as for compound 4 afforded crude methyl ketal 13 (0.03 g, 97% from 9). An analytical sample purified by column chromatography on silica gel had 1H and 13C NMR spectra identical to those of an authentic standard prepared from 18-hydroxyprogesterone.20

Acknowledgment. We thank Universidad de Buenos Aires and CONICET (Argentina) for financial support of this work. Supporting Information Available: 1H and 13C NMR spectra of 3, 5, and 6 and NMR peak assignments for compounds 3, 4, 5, 6, 7, and 12 (9 pages). This material is contained in libraries on microfiche, immediately follows this article in the microfilm version of the journal, and can be ordered from the ACS; see any current masthead page for ordering information. JO960108Q (19) Pappo, R. U.S. Patent 3 017 410, 1962; Chem. Abstr. 1962, 56, 12988h. (20) Pappo, R. U.S. Patent 2 907 758, 1959; Chem. Abstr. 1960, 54, 2431d.